1. Field of the Invention
The present invention relates to an optical information recording medium having a plurality of information recording layers. The present invention also relates to an information reproduction apparatus for reproducing data from such information recording medium. Furthermore, the present invention relates to an information recording apparatus for recording information on such information recording medium.
2. Description of the Related Art
In recent years, a DVD having a capacity about 7 times that of a CD (compact disk) has been developed, and has become prevalent. DVD disks include read-only DVD-ROM disk, write-once DVD-R disk, rewritable DVD-RAM disk, and the like depending on their characteristics. The DVD disk has a lead-in zone that records sub information in addition to a data zone used to record main information.
JIS X 6243, which specifies the DVD-RAM disk, specifies a data zone and a lead-in zone which is located on the inner periphery side of the data zone. The data zone is a data-rewritable main information recording area. On the other hand, the lead-in zone is an area wherein sub information is recorded by embossed prepit trains. The sub information includes information of a disk structure, recording/reproduction parameters, and the like.
JIS X 6241, which specifies the DVD-ROM disk, specifies a disk having two layers per side. The disk having two layers per side has a larger capacity per side than a disk having one layer per side. In recent years, not only the ROM disk, but also a two-layered rewritable disk has been studied.
In order to realize a two-layered rewritable disk, the influence from the other layer (a layer on the back or front side viewed from the objective lens side) must be reduced. For example, Jpn. Pat. Appln. KOKAI Publication No. 2000-36130 has proposed a method of reducing such influence. Jpn. Pat. Appln. KOKAI Publication No. 2000-36130 describes that the relationship between the reflectance and light absorbency of first and second layers is limited, and the recording order with respect to the first and second layers is limited. In the recording order, information is recorded first on the first layer which is located on the front side viewed from the objective lens side, and is then on the second layer which is located on the back side viewed from the objective lens side. With such limitations, the influence from the other layer can be reduced, and recording/reproduction can be implemented.
However, upon realizing a two-layered rewritable disk, the influence from the other layer due to a decentering error poses another problem. The lead-in zone and data zone have different light reflectances and absorbencies. If the disk is free from any decentering errors, the lead-in zone and data zone in the first layer are formed at the same radial positions as those in the second layer. Therefore, a preventive measure can be taken while predicting influences from the other layer to some extent.
However, it is nearly impossible to manufacture a disk free from any decentering errors. For this reason, the radial positions of the lead-in zone and data zone in the first layer become slightly different from those in the second layer. Such slight difference seriously influences information reproduced near the boundary of the lead-in zone and data zone, resulting in unstable reproduction.
Jpn. Pat. Appln. KOKAI Publication No. 62-285232 discloses an optical disk in which guard areas which do not undergo any data recording/reproduction are formed before and after the boundary between a data read-only area and data recording area. However, this disk does not consider any preventive measure against decentering errors, and the above technique cannot solve the aforementioned problems.